reverseproxy: do not parse upstream address too early if it contains replaceble parts (#5695)

* reverseproxy: do not parse upstream address too early if it contains replaceble parts

* remove unused method

* cleanup

* accommodate partially replaceable port

7 files changed, 365 insertions, 40 deletions

diff --git a/caddytest/integration/caddyfile_adapt/replaceable_upstream.txt b/caddytest/integration/caddyfile_adapt/replaceable_upstream.txt
new file mode 100644
index 000000000..202e33043
--- /dev/null
+++ b/caddytest/integration/caddyfile_adapt/replaceable_upstream.txt
@@ -0,0 +1,100 @@
+*.sandbox.localhost {
+	@sandboxPort {
+		header_regexp first_label Host ^([0-9]{3})\.sandbox\.
+	}
+	handle @sandboxPort {
+		reverse_proxy {re.first_label.1}
+	}
+	handle {
+		redir {scheme}://application.localhost
+	}
+}
+
+----------
+{
+	"apps": {
+		"http": {
+			"servers": {
+				"srv0": {
+					"listen": [
+						":443"
+					],
+					"routes": [
+						{
+							"match": [
+								{
+									"host": [
+										"*.sandbox.localhost"
+									]
+								}
+							],
+							"handle": [
+								{
+									"handler": "subroute",
+									"routes": [
+										{
+											"group": "group2",
+											"handle": [
+												{
+													"handler": "subroute",
+													"routes": [
+														{
+															"handle": [
+																{
+																	"handler": "reverse_proxy",
+																	"upstreams": [
+																		{
+																			"dial": "{http.regexp.first_label.1}"
+																		}
+																	]
+																}
+															]
+														}
+													]
+												}
+											],
+											"match": [
+												{
+													"header_regexp": {
+														"Host": {
+															"name": "first_label",
+															"pattern": "^([0-9]{3})\\.sandbox\\."
+														}
+													}
+												}
+											]
+										},
+										{
+											"group": "group2",
+											"handle": [
+												{
+													"handler": "subroute",
+													"routes": [
+														{
+															"handle": [
+																{
+																	"handler": "static_response",
+																	"headers": {
+																		"Location": [
+																			"{http.request.scheme}://application.localhost"
+																		]
+																	},
+																	"status_code": 302
+																}
+															]
+														}
+													]
+												}
+											]
+										}
+									]
+								}
+							],
+							"terminal": true
+						}
+					]
+				}
+			}
+		}
+	}
+}
\ No newline at end of file
diff --git a/caddytest/integration/caddyfile_adapt/replaceable_upstream_partial_port.txt b/caddytest/integration/caddyfile_adapt/replaceable_upstream_partial_port.txt
new file mode 100644
index 000000000..7fbcb5c78
--- /dev/null
+++ b/caddytest/integration/caddyfile_adapt/replaceable_upstream_partial_port.txt
@@ -0,0 +1,100 @@
+*.sandbox.localhost {
+	@sandboxPort {
+		header_regexp port Host ^([0-9]{3})\.sandbox\.
+	}
+	handle @sandboxPort {
+		reverse_proxy app:6{re.port.1}
+	}
+	handle {
+		redir {scheme}://application.localhost
+	}
+}
+
+----------
+{
+	"apps": {
+		"http": {
+			"servers": {
+				"srv0": {
+					"listen": [
+						":443"
+					],
+					"routes": [
+						{
+							"match": [
+								{
+									"host": [
+										"*.sandbox.localhost"
+									]
+								}
+							],
+							"handle": [
+								{
+									"handler": "subroute",
+									"routes": [
+										{
+											"group": "group2",
+											"handle": [
+												{
+													"handler": "subroute",
+													"routes": [
+														{
+															"handle": [
+																{
+																	"handler": "reverse_proxy",
+																	"upstreams": [
+																		{
+																			"dial": "app:6{http.regexp.port.1}"
+																		}
+																	]
+																}
+															]
+														}
+													]
+												}
+											],
+											"match": [
+												{
+													"header_regexp": {
+														"Host": {
+															"name": "port",
+															"pattern": "^([0-9]{3})\\.sandbox\\."
+														}
+													}
+												}
+											]
+										},
+										{
+											"group": "group2",
+											"handle": [
+												{
+													"handler": "subroute",
+													"routes": [
+														{
+															"handle": [
+																{
+																	"handler": "static_response",
+																	"headers": {
+																		"Location": [
+																			"{http.request.scheme}://application.localhost"
+																		]
+																	},
+																	"status_code": 302
+																}
+															]
+														}
+													]
+												}
+											]
+										}
+									]
+								}
+							],
+							"terminal": true
+						}
+					]
+				}
+			}
+		}
+	}
+}
\ No newline at end of file
diff --git a/caddytest/integration/caddyfile_adapt/replaceable_upstream_port.txt b/caddytest/integration/caddyfile_adapt/replaceable_upstream_port.txt
new file mode 100644
index 000000000..8f75c5bd0
--- /dev/null
+++ b/caddytest/integration/caddyfile_adapt/replaceable_upstream_port.txt
@@ -0,0 +1,100 @@
+*.sandbox.localhost {
+	@sandboxPort {
+		header_regexp port Host ^([0-9]{3})\.sandbox\.
+	}
+	handle @sandboxPort {
+		reverse_proxy app:{re.port.1}
+	}
+	handle {
+		redir {scheme}://application.localhost
+	}
+}
+
+----------
+{
+	"apps": {
+		"http": {
+			"servers": {
+				"srv0": {
+					"listen": [
+						":443"
+					],
+					"routes": [
+						{
+							"match": [
+								{
+									"host": [
+										"*.sandbox.localhost"
+									]
+								}
+							],
+							"handle": [
+								{
+									"handler": "subroute",
+									"routes": [
+										{
+											"group": "group2",
+											"handle": [
+												{
+													"handler": "subroute",
+													"routes": [
+														{
+															"handle": [
+																{
+																	"handler": "reverse_proxy",
+																	"upstreams": [
+																		{
+																			"dial": "app:{http.regexp.port.1}"
+																		}
+																	]
+																}
+															]
+														}
+													]
+												}
+											],
+											"match": [
+												{
+													"header_regexp": {
+														"Host": {
+															"name": "port",
+															"pattern": "^([0-9]{3})\\.sandbox\\."
+														}
+													}
+												}
+											]
+										},
+										{
+											"group": "group2",
+											"handle": [
+												{
+													"handler": "subroute",
+													"routes": [
+														{
+															"handle": [
+																{
+																	"handler": "static_response",
+																	"headers": {
+																		"Location": [
+																			"{http.request.scheme}://application.localhost"
+																		]
+																	},
+																	"status_code": 302
+																}
+															]
+														}
+													]
+												}
+											]
+										}
+									]
+								}
+							],
+							"terminal": true
+						}
+					]
+				}
+			}
+		}
+	}
+}
\ No newline at end of file
diff --git a/modules/caddyhttp/reverseproxy/addresses.go b/modules/caddyhttp/reverseproxy/addresses.go
index 6078f1194..093949739 100644
--- a/modules/caddyhttp/reverseproxy/addresses.go
+++ b/modules/caddyhttp/reverseproxy/addresses.go
@@ -23,11 +23,43 @@ import (
 	"github.com/caddyserver/caddy/v2"
 )
 
+type parsedAddr struct {
+	network, scheme, host, port string
+	valid                       bool
+}
+
+func (p parsedAddr) dialAddr() string {
+	if !p.valid {
+		return ""
+	}
+	// for simplest possible config, we only need to include
+	// the network portion if the user specified one
+	if p.network != "" {
+		return caddy.JoinNetworkAddress(p.network, p.host, p.port)
+	}
+
+	// if the host is a placeholder, then we don't want to join with an empty port,
+	// because that would just append an extra ':' at the end of the address.
+	if p.port == "" && strings.Contains(p.host, "{") {
+		return p.host
+	}
+	return net.JoinHostPort(p.host, p.port)
+}
+func (p parsedAddr) rangedPort() bool {
+	return strings.Contains(p.port, "-")
+}
+func (p parsedAddr) replaceablePort() bool {
+	return strings.Contains(p.port, "{") && strings.Contains(p.port, "}")
+}
+func (p parsedAddr) isUnix() bool {
+	return caddy.IsUnixNetwork(p.network)
+}
+
 // parseUpstreamDialAddress parses configuration inputs for
 // the dial address, including support for a scheme in front
 // as a shortcut for the port number, and a network type,
 // for example 'unix' to dial a unix socket.
-func parseUpstreamDialAddress(upstreamAddr string) (string, string, error) {
+func parseUpstreamDialAddress(upstreamAddr string) (parsedAddr, error) {
 	var network, scheme, host, port string
 
 	if strings.Contains(upstreamAddr, "://") {
@@ -35,7 +67,7 @@ func parseUpstreamDialAddress(upstreamAddr string) (string, string, error) {
 		// so we return a more user-friendly error message instead
 		// to explain what to do instead
 		if strings.Contains(upstreamAddr, "{") {
-			return "", "", fmt.Errorf("due to parsing difficulties, placeholders are not allowed when an upstream address contains a scheme")
+			return parsedAddr{}, fmt.Errorf("due to parsing difficulties, placeholders are not allowed when an upstream address contains a scheme")
 		}
 
 		toURL, err := url.Parse(upstreamAddr)
@@ -46,19 +78,19 @@ func parseUpstreamDialAddress(upstreamAddr string) (string, string, error) {
 			if strings.Contains(err.Error(), "invalid port") && strings.Contains(err.Error(), "-") {
 				index := strings.LastIndex(upstreamAddr, ":")
 				if index == -1 {
-					return "", "", fmt.Errorf("parsing upstream URL: %v", err)
+					return parsedAddr{}, fmt.Errorf("parsing upstream URL: %v", err)
 				}
 				portRange := upstreamAddr[index+1:]
 				if strings.Count(portRange, "-") != 1 {
-					return "", "", fmt.Errorf("parsing upstream URL: parse \"%v\": port range invalid: %v", upstreamAddr, portRange)
+					return parsedAddr{}, fmt.Errorf("parsing upstream URL: parse \"%v\": port range invalid: %v", upstreamAddr, portRange)
 				}
 				toURL, err = url.Parse(strings.ReplaceAll(upstreamAddr, portRange, "0"))
 				if err != nil {
-					return "", "", fmt.Errorf("parsing upstream URL: %v", err)
+					return parsedAddr{}, fmt.Errorf("parsing upstream URL: %v", err)
 				}
 				port = portRange
 			} else {
-				return "", "", fmt.Errorf("parsing upstream URL: %v", err)
+				return parsedAddr{}, fmt.Errorf("parsing upstream URL: %v", err)
 			}
 		}
 		if port == "" {
@@ -69,18 +101,18 @@ func parseUpstreamDialAddress(upstreamAddr string) (string, string, error) {
 		// a backend and proxying to it, so we cannot allow extra components
 		// in backend URLs
 		if toURL.Path != "" || toURL.RawQuery != "" || toURL.Fragment != "" {
-			return "", "", fmt.Errorf("for now, URLs for proxy upstreams only support scheme, host, and port components")
+			return parsedAddr{}, fmt.Errorf("for now, URLs for proxy upstreams only support scheme, host, and port components")
 		}
 
 		// ensure the port and scheme aren't in conflict
 		if toURL.Scheme == "http" && port == "443" {
-			return "", "", fmt.Errorf("upstream address has conflicting scheme (http://) and port (:443, the HTTPS port)")
+			return parsedAddr{}, fmt.Errorf("upstream address has conflicting scheme (http://) and port (:443, the HTTPS port)")
 		}
 		if toURL.Scheme == "https" && port == "80" {
-			return "", "", fmt.Errorf("upstream address has conflicting scheme (https://) and port (:80, the HTTP port)")
+			return parsedAddr{}, fmt.Errorf("upstream address has conflicting scheme (https://) and port (:80, the HTTP port)")
 		}
 		if toURL.Scheme == "h2c" && port == "443" {
-			return "", "", fmt.Errorf("upstream address has conflicting scheme (h2c://) and port (:443, the HTTPS port)")
+			return parsedAddr{}, fmt.Errorf("upstream address has conflicting scheme (h2c://) and port (:443, the HTTPS port)")
 		}
 
 		// if port is missing, attempt to infer from scheme
@@ -112,18 +144,5 @@ func parseUpstreamDialAddress(upstreamAddr string) (string, string, error) {
 		network = "unix"
 		scheme = "h2c"
 	}
-
-	// for simplest possible config, we only need to include
-	// the network portion if the user specified one
-	if network != "" {
-		return caddy.JoinNetworkAddress(network, host, port), scheme, nil
-	}
-
-	// if the host is a placeholder, then we don't want to join with an empty port,
-	// because that would just append an extra ':' at the end of the address.
-	if port == "" && strings.Contains(host, "{") {
-		return host, scheme, nil
-	}
-
-	return net.JoinHostPort(host, port), scheme, nil
+	return parsedAddr{network, scheme, host, port, true}, nil
 }
diff --git a/modules/caddyhttp/reverseproxy/addresses_test.go b/modules/caddyhttp/reverseproxy/addresses_test.go
index 0c7ad7b31..0c5141942 100644
--- a/modules/caddyhttp/reverseproxy/addresses_test.go
+++ b/modules/caddyhttp/reverseproxy/addresses_test.go
@@ -265,18 +265,18 @@ func TestParseUpstreamDialAddress(t *testing.T) {
 			expectScheme:   "h2c",
 		},
 	} {
-		actualHostPort, actualScheme, err := parseUpstreamDialAddress(tc.input)
+		actualAddr, err := parseUpstreamDialAddress(tc.input)
 		if tc.expectErr && err == nil {
 			t.Errorf("Test %d: Expected error but got %v", i, err)
 		}
 		if !tc.expectErr && err != nil {
 			t.Errorf("Test %d: Expected no error but got %v", i, err)
 		}
-		if actualHostPort != tc.expectHostPort {
-			t.Errorf("Test %d: Expected host and port '%s' but got '%s'", i, tc.expectHostPort, actualHostPort)
+		if actualAddr.dialAddr() != tc.expectHostPort {
+			t.Errorf("Test %d: input %s: Expected host and port '%s' but got '%s'", i, tc.input, tc.expectHostPort, actualAddr.dialAddr())
 		}
-		if actualScheme != tc.expectScheme {
-			t.Errorf("Test %d: Expected scheme '%s' but got '%s'", i, tc.expectScheme, actualScheme)
+		if actualAddr.scheme != tc.expectScheme {
+			t.Errorf("Test %d: Expected scheme '%s' but got '%s'", i, tc.expectScheme, actualAddr.scheme)
 		}
 	}
 }
diff --git a/modules/caddyhttp/reverseproxy/caddyfile.go b/modules/caddyhttp/reverseproxy/caddyfile.go
index 1d86bebd7..f5eb7a5d0 100644
--- a/modules/caddyhttp/reverseproxy/caddyfile.go
+++ b/modules/caddyhttp/reverseproxy/caddyfile.go
@@ -146,7 +146,7 @@ func (h *Handler) UnmarshalCaddyfile(d *caddyfile.Dispenser) error {
 	// appendUpstream creates an upstream for address and adds
 	// it to the list.
 	appendUpstream := func(address string) error {
-		dialAddr, scheme, err := parseUpstreamDialAddress(address)
+		pa, err := parseUpstreamDialAddress(address)
 		if err != nil {
 			return d.WrapErr(err)
 		}
@@ -154,21 +154,27 @@ func (h *Handler) UnmarshalCaddyfile(d *caddyfile.Dispenser) error {
 		// the underlying JSON does not yet support different
 		// transports (protocols or schemes) to each backend,
 		// so we remember the last one we see and compare them
-		if commonScheme != "" && scheme != commonScheme {
+		if commonScheme != "" && pa.scheme != commonScheme {
 			return d.Errf("for now, all proxy upstreams must use the same scheme (transport protocol); expecting '%s://' but got '%s://'",
-				commonScheme, scheme)
+				commonScheme, pa.scheme)
 		}
-		commonScheme = scheme
+		commonScheme = pa.scheme
 
-		parsedAddr, err := caddy.ParseNetworkAddress(dialAddr)
+		// if the port of upstream address contains a placeholder, only wrap it with the `Upstream` struct,
+		// delaying actual resolution of the address until request time.
+		if pa.replaceablePort() {
+			h.Upstreams = append(h.Upstreams, &Upstream{Dial: pa.dialAddr()})
+			return nil
+		}
+		parsedAddr, err := caddy.ParseNetworkAddress(pa.dialAddr())
 		if err != nil {
 			return d.WrapErr(err)
 		}
 
-		if parsedAddr.StartPort == 0 && parsedAddr.EndPort == 0 {
+		if pa.isUnix() || !pa.rangedPort() {
 			// unix networks don't have ports
 			h.Upstreams = append(h.Upstreams, &Upstream{
-				Dial: dialAddr,
+				Dial: pa.dialAddr(),
 			})
 		} else {
 			// expand a port range into multiple upstreams
diff --git a/modules/caddyhttp/reverseproxy/command.go b/modules/caddyhttp/reverseproxy/command.go
index 48fabd56d..8438b7278 100644
--- a/modules/caddyhttp/reverseproxy/command.go
+++ b/modules/caddyhttp/reverseproxy/command.go
@@ -132,14 +132,14 @@ func cmdReverseProxy(fs caddycmd.Flags) (int, error) {
 	toAddresses := make([]string, len(to))
 	var toScheme string
 	for i, toLoc := range to {
-		addr, scheme, err := parseUpstreamDialAddress(toLoc)
+		addr, err := parseUpstreamDialAddress(toLoc)
 		if err != nil {
 			return caddy.ExitCodeFailedStartup, fmt.Errorf("invalid upstream address %s: %v", toLoc, err)
 		}
-		if scheme != "" && toScheme == "" {
-			toScheme = scheme
+		if addr.scheme != "" && toScheme == "" {
+			toScheme = addr.scheme
 		}
-		toAddresses[i] = addr
+		toAddresses[i] = addr.dialAddr()
 	}
 
 	// proceed to build the handler and server